The natural world is built upon a division between prokaryotic and eukaryotic cells. Prokaryotes, such as bacteria and archaea, are microscopic, single-celled organisms defined by the absence of a nucleus. In contrast, most visible life is eukaryotic and multicellular, with complex cells that house their DNA within a nucleus. This distinction makes the concept of a “multicellular prokaryote” seem contradictory, yet some prokaryotes challenge this division. These organisms exist as coordinated, collective units that function like a single, larger entity, representing a form of primitive multicellularity.
What Constitutes Prokaryotic Multicellularity?
Prokaryotic multicellularity is more than a simple aggregation of cells, such as those in a biofilm. It involves specific criteria that elevate a group of cells into a cooperative, singular organism. A primary indicator is that the cells remain physically connected following division, forming chains or other structured arrangements.
A defining feature of this arrangement is the functional specialization, or differentiation, of cells within the group. Certain cells adopt specific roles that benefit the entire collective, even if it means sacrificing their own reproductive potential. This division of labor requires a system of intercellular communication, where cells exchange chemical signals to coordinate their activities.
This level of organization allows the group to achieve feats that would be impossible for individual cells. The developmental programs that guide this differentiation, though simpler than in eukaryotes, are consistent and reproducible.
Notable Examples of Multicellular Prokaryotes
Among the most studied examples of multicellular prokaryotes are certain species of cyanobacteria. The genus Anabaena, for instance, forms long, filamentous chains of cells. Under conditions of nitrogen scarcity, specific cells in this chain transform. These cells, called heterocysts, cease photosynthesis and dedicate themselves to fixing atmospheric nitrogen into ammonia, which they share with neighboring photosynthetic cells.
This division of labor creates a self-sufficient filament capable of thriving where individual cells could not. The heterocysts, with their thickened cell walls that block oxygen, provide the anaerobic environment necessary for nitrogen fixation. In return, the adjacent vegetative cells supply the heterocysts with carbon compounds produced through photosynthesis.
Another compelling case is found in the myxobacteria, soil-dwelling microbes known for their highly social behavior. These bacteria swarm together in coordinated packs to hunt other microorganisms. When food becomes scarce, myxobacteria initiate a developmental process where thousands of cells aggregate to construct a complex structure known as a fruiting body.
Within this structure, a division of labor occurs. Some bacteria differentiate into dormant myxospores, which are resistant to harsh environmental conditions. Other cells sacrifice themselves to form the stalk of the fruiting body, lifting the spores to a position where they are more likely to be dispersed to a new, favorable location.
Cooperative Behaviors and Cell Communication
The complex, coordinated activities in multicellular prokaryotes depend on constant communication between the cells. This dialogue is primarily chemical, involving the release and detection of signaling molecules that inform cells about their environment and the status of their neighbors. These signals enable the population to act as a cohesive unit.
In myxobacteria, this communication is exemplified by quorum sensing. As the population grows, individual cells secrete signaling molecules, and the concentration serves as a proxy for population density. When the concentration reaches a certain threshold and nutrients are scarce, it triggers the aggregation and fruiting body formation process.
A different form of communication underpins cooperation in cyanobacterial filaments like Anabaena. The relationship between the nitrogen-fixing heterocysts and photosynthetic cells is maintained through a direct exchange of metabolic products. This direct pipeline of nutrients acts as an ongoing communication system, ensuring the metabolic activities of the different cell types remain balanced.
Key Differences from Eukaryotic Multicellularity
While prokaryotes have evolved forms of multicellularity, these systems differ significantly from the complex multicellularity seen in eukaryotes like animals and plants. A prominent distinction lies in the degree and stability of cell differentiation. In prokaryotes such as Anabaena, the specialization of a heterocyst is often reversible. In contrast, the specialized cells in an animal, such as a neuron, are terminally differentiated and cannot revert to a less specialized state.
Prokaryotic multicellularity also does not lead to the formation of complex tissues and organs. A cyanobacterial filament or a myxobacterial fruiting body are intricate structures, but they lack the hierarchical organization of specialized cells grouped into tissues, which in turn form organs with distinct physiological functions. This level of structural complexity is a hallmark of eukaryotic life.
Another key difference relates to reproduction. In most multicellular prokaryotes, nearly any cell within the collective can potentially detach and give rise to a new, independent colony. This contrasts with the reproductive strategy of many eukaryotes, which have a strict separation between a protected, reproductive germline and the non-reproductive somatic cells that constitute the rest of the body.